The present invention relates to high-molecular-weight copolymers comprising ethylene and propylene units which have an ethylene content in the range from 1 to 10% by weight.
The invention also relates to a process for the preparation of these polymers, and to moldings, such as pipes, fittings, hollowware and sheets, made from said polymers.
DE-A-40 19 053 discloses homopolymers having a broad molecular-weight distribution. These homopolymers can be converted into pipes with great difficulty. However, pipes produced in this way have the disadvantage of being very brittle and having a rough surface, which means that they are not suitable for practical use.
EP-A-573 862 discloses a process for the preparation of polypropylene having a molecular-weight distribution Mw/Mn of  greater than 20 and good processing properties. The melt flow index is 2 dg/min; the intrinsic viscosity is 280 ml/g. The polypropylene described in this way is prepared by gas-phase polymerization. Examples 1 to 4 of EP-A-573 862 describe the preparation of a homopolypropylene powder having a broad molecular-weight distribution. Although none of the examples indicate the polydispersity Mw/Mn, the intrinsic viscosity data (800 ml/g and 67 ml/g) suggest a very broad molecular-weight spread in the first and second steps.
The processes described in the prior art (EP-A-573 862) have been repeated in order to enable testing of the properties of the materials. It has been found that all raw materials are very brittle and have restricted processing quality and material inhomogeneity. The production of PP pipes by a conventional extrusion process was in some cases impossible since the viscosity of the melt was inadequate for an extrusion process.
The object of the present invention was to find an improved molding composition which allows pipes having low brittleness and a smooth surface and in addition high toughness and good rigidity in combination with excellent creep rupture strength to be produced using conventional production tools.
This object is achieved by copolymers of the generic type mentioned at the outset which have a melt flow rate MFR (230/5) of  less than 2 dg/min and a molecular-weight distribution Mw/Mn in the range from 6 to 20.
Surprisingly, it has been found that the novel propylene-ethylene copolymers can be converted, using conventional production tools, into pipes which have smooth finished surfaces, good processing quality, high impact strength, good hardness and good creep rupture strength.
The invention also relates to a process for the preparation of the propylene/ethylene copolymers by copolymerization of propylene and ethylene, if desired with a further 1-olefin having 4 to 20 carbon atoms, in suspension at a temperature in the range from 30 to 150xc2x0 C., a pressure of from 10 to 100 bar and a residence time of from 30 min to 6 h, in the presence of a commercially available catalyst (for example catalyst FT4S from Montell, Milan, Italy wherein FT 4 S comprises a titanium halide and an electron donor supported on an active MgCl2), an organoaluminum compound (B) and, if desired, an organosilicon compound (C), which comprises carrying out the polymerization in two reaction steps, where the suspension medium in the first step is both monomer and suspension medium and a polypropylene having a viscosity of from 500 to 1400 ml/g which makes up a proportion of from 20 to 80% of the total polymer is prepared in the first reaction step, and where the total polymer after the second reaction step has a viscosity of from 400 to 700 ml/g and a polydispersity Mw/Mn of from 6 to 20.
In the first reaction step, a high-molecular-weight product having a viscosity of from 500 to 1400 ml/g which makes up a proportion of from 20 to 80% by weight, preferably from 45 to 75% by weight, particularly preferably from 48 to 65% by weight, of the total polymer is prepared, while in the second reaction step, a low-molecular-weight product having a viscosity of from 200 to 400 ml/g which makes up a proportion of from 80 to 20% by weight, preferably from 55 to 25% by weight, particularly preferably from 52 to 35% by weight, is prepared.
The polymerization is carried out by a bulk process in two reaction steps, where the monomer, the propylene, is simultaneously starting material and suspension medium.
The novel process is carried out as a two-step polymerization with prior prepolymerization. Both the first and the second reaction steps and the prepolymerization can be carried out either batchwise or continuously. Continuous mode is preferred.
Component B and component C are mixed with one another before the prepolymerization and then brought into contact with the catalyst. Propylene is prepolymerized in suspension or in bulk in the presence of these active components. The prepolymerization is preferably carried out in the liquid monomer. The residence time is from 4 to 10 minutes, and the prepolymerization temperature is in the range from 10 to 25xc2x0 C.
The prepolymer is then introduced into the first reaction step of the polymerization, where it is polymerized in liquid propylene at a temperature of from 55 to 100xc2x0 C. and at a residence time of from 0.5 to 3.5 h. A phase ratio in the range from 2.5 to 4 l of liquid propylene per kg of PP, preferably of 3.3 l of liquid propylene per kg of PP, is established. In the first reaction step, ethylene is metered in continuously in such a way that a C2 concentration in the liquid phase of from 0.1 to 20% by weight, preferably from 0.1 to 10% by weight, is established. Hydrogen is metered in for molecular-weight regulation.
After the first reaction step, the multiphase system is transferred into the second reaction step, where it is polymerized at a temperature of from 55 to 100xc2x0 C. The second reaction step is carried out in a second reactor, where a phase ratio of from 1 to 2.5 l of liquid propylene per kg of PP, preferably of 1.9 l of liquid propylene per kg of PP, is established. It is preferred in accordance with the invention to establish different phase ratios in the two reactors in the process described here. As described above, ethylene and H2 are likewise metered in.
The temperatures, hydrogen concentrations and ethylene concentrations in the two reactors can be identical or different. Suitable reactors are stirred reactors or loop reactors.
It is possible to decompress the monomer between the two reactors and to meter the catalyst/PP system, which is still polymerization-active, into the second reactor. It is also possible to set a lower hydrogen concentration in the second reactor than in the first reactor.
Component B is trimethylaluminum, triisobutylaluminum or triethylaluminum. Triethylaluminum or triisobutylaluminum is preferred. Triethylaluminum is particularly preferred.
Component C is cyclohexylmethyldimethoxysilane, biscyclopentyldimethoxysilane or diphenyldimethoxysilane. Cyclohexylmethyldimethoxysilane or biscyclopentyidimethoxysilane is particularly preferred.
Component B is employed in a concentration of from 0.001 to 10 mmol/l, preferably from 0.1 to 5 mmol/l. Component C is employed in a ratio R with respect to component B. The ratio is calculated from the quotient of concentration B and concentration C, in each case in mol/l. The ratio is from 1 to 200, preferably from 2 to 100, particularly preferably from 2.5 to 75.
Preference is given in accordance with the invention to products having an MFR (230/5) of 0.01 to 5 dg/min, particularly preferably from 0.02 to 2 dg/min. The copolymer of the invention consists of from 1.0 to 10% by weight of ethylene units and from 99 to 90% by weight of propylene units.
After the second reaction step, the mixture of propylene, hydrogen and ethylene is worked up. Preference is given to rapid evaporation of the liquid monomer in one step. The purified copolymer is subsequently dried in a stream of inert gas, and it is ensured that the copolymer is free from monomer. The resultant high-molecular-weight copolymer is mixed with stabilizers, lubricants, fillers, pigments, etc. and granulated. The granulation is carried out in an extruder or compounder.
The evaporated monomer mixture is condensed and separated by distillation into ethylene, propylene and hydrogen. The distillation should be designed so that a hydrogen concentration of  less than 150 ppm, preferably  less than 80 ppm, is ensured. The monomer purified in this way is then metered back into the first reactor.